Most people are terrible at shuffling cards. The two main ways people shuffle are:

A. Really big riffle which exposes all the cards, and then bends the cards when you put them back together.

B. Just chunk them into your hands, not really doing a whole lot.

Both ways expose the cards so people have an idea where certain cards are (or are not), and ruin your deck of cards much faster.

To make the cards completely random, it is a good idea to wash the cards on the table first (spread them all out, mix them around with your hands), then shuffle. Including the wash, this is the method used by Casino Dealers (when they aren't using the auto shuffle).

You may be asking why bother? When cards are exposed during the shuffle, then people can keep basic track of them. They might not know exactly where it is, but they can have a reasonable idea of whether or not the card will be dealt during the hand.